The present invention generally relates to semiconductor integrated circuits, and more particularly to a semiconductor integrated circuit with reduced power consumption.
FIG. 1 shows an example of the conventional synthesizer tuner. The synthesizer tuner comprises a high frequency amplifier 11, a frequency mixer 12, an intermediate frequency amplifier 13, a frequency demodulator 14, a stereo demondulator 15, low frequency demodulators 16 and 17, and an oscillator circuit 20. The oscillator circuit 20 comprises a local oscillator 21, a prescaler 22, a programmable counter 23, a crystal oscillator 24, a frequency divider 25, a phase comparator 26, a low pass filter 27, and a voltage control circuit 28.
A very high frequency (VHF) signal received by an antenna 10 is amplified in the high frequency amplifier 11 and supplied to the frequency mixer 12. A voltage from the oscillator circuit 20 is supplied to the frequency mixer 12, and the frequency mixer 12 outputs an intermediate frequency signal corresponding to a difference between the frequency of the received signal and the oscillation frequency of the oscillator circuit 20. The intermediate frequency signal is amplified in the intermediate frequency amplifier 13 and supplied to the frequency demodulator 14 which frequency-demodulates the intermediate frequency signal. A demodulated output signal of the frequency demodulator 14 is supplied to the stereo demondulator 15 which separates the demodulated output signal into right-channel and left-channel signals. The right-channel signal is amplified in the low frequency amplifier 16 and outputted through an output terminal 18, while the left-channel signal is amplified in the low frequency amplifier 17 and outputted through an output terminal 19.
The voltage control circuit 28 of the oscillator circuit 20 is provided to reduce the power consumption of the prescaler circuit 22. An output signal of the local oscillator 21 is frequency-divided in the prescaler circuit 22, and the prescaler circuit 22 supplies a frequency divided signal to the programmable counter 23. A predetermined counted value N is set in the programmable counter 23. An output signal of the programmable counter 23 is supplied to the phase comparator 26. On the other hand, an output signal of the crystal oscillator 24 is frequency-divided in the frequency divider 25 and supplied to the phase comparator 26. The phase comparator 26 compares the phase of the output signal of the programmable counter 23 and the phase of the output signal of the frequency divider 25, and outputs a phase error signal dependent on the phase error between the two signals. The phase comparator 26 uses the output phase error signal thereof to control the oscillation frequency of the local oscillator 21 through the lowpass filter 27.
When the phases of the two signals compared in the phase comparator 26 coincide, the synthesizer tuner operates only responsive to the output signal of the local oscillator 21, and the prescaler 22 is substantially unused. In the case, the phase comparator 26 supplies to the voltage control circuit 28 a signal which indicates that the phases of the two compared signals coincide, and the voltage control circuit 28 cuts off the supply of the voltage to the prescaler 22. The operation speeds of the programmable counter 23 and the phase comparator 26 are slow compared to the operation speed of the prescaler 22. For this reason, it takes a few milliseconds for the phase error to occur after the supply of voltage to the prescaler 22 is cut off.
When the phase error occurs, the voltage control circuit 28 is operated responsive to the output signal of the phase comparator 26. As a result, the voltage is supplied to the prescaler 22 from the voltage control circuit 28, and the phase comparison is normally carried out in the phase comparator 26. Generally, an OFF time period in which the supply of voltage to the prescaler 22 is cut off is set to a value in the order of 1/5 an ON time period in which the supply of voltage to the prescaler 22 is made, so that the tuning error is negligible to the listener.
However, according to the conventional oscillator circuit 20 in which the voltage control circuit 28 supplies a reduced small voltage to the prescaler 22 so as to reduce the power consumption, it requires a large voltage to return the prescaler 22 and a large driving capacity is required of the voltage control circuit 28 in order to increase the voltage supplied to the prescaler 22. For this reason, the circuit scale of the voltage control circuit 28 inevitably becomes large. In addition, the circuit construction of the voltage control circuit 28 becomes complex because the voltage setting after the prescaler 22 is returned must be made with a high accuracy.